I looked at the text of the Budapest Memorandum. The main commitments seem to be a commitment to not use force against Ukraine, and a commitment to "seek immediate United Nations Security Council action to provide assistance to Ukraine... if Ukraine should become a victim of an act of aggression".
I absolutely hope that every country supports Ukraine and helps them at the moment. I'm just saying that it seems to me that Russia is the one who has abandoned its commitments within the Budapest Memorandum. I don't know if you can say that the US and the UK have (Wikipedia says that France and China gave assurances in separate documents, not in the Budapest Memorandum). Although I absolutely hope that the US will take a more pro-Ukraine stance as soon as possible.
I'm talking about what was actually agreed to. To me it seems that Russia quite clearly abandoned its commitments within the Budapest Memorandum. I don't think you can say that the US and the UK did, unless you're saying that those two countries didn't do enough within the UN Security Council to back Ukraine.
Surely the primary country to blame for this situation is Russia. Russia invaded Ukraine in 2014. Western countries didn't invade Ukraine.